1. Field of the Invention
The present invention relates to a gas discharge tube in which a tunnel effect type electron emitting device is used for a cathode, and a method for manufacturing such a cathode.
2. Description of the Prior Art
Conventional tunnel effect type electron emitting device such as one disclosed in Japanese Patent Publication No. 43-3041 is primarily intended to be used for a vacuum tube. Such an electron emitting device has a sandwich structure as shown in FIG. 8, which is an M-I-M structure composed of a metal base layer 2, a dielectric film 3 having a thickness of 100 .ANG., and a surface metal layer 4 having a thickness of 80 .ANG. to 300 .ANG. deposited in this order on a glass substrate 1. An electron emission area 5 is formed on a center portion of the surface metal layer 4.
Such an electron emitting device is disposed in a vacuum airtight valve in such a manner that one surface of the electron emitting device on which the above layers are deposited faces an anode. When a DC voltage V.sub.d is applied between the metal base layer 2 and the surface metal layer 4 with the latter being positive, an energy level between the layers as illustratively shown in FIG. 9 is obtained. In this illustration, .phi..sub.A, .phi..sub.B, and .phi..sub.M are potential barriers and eV.sub.d is potential energy caused by the applied DC voltage V.sub.d (e: elementary electric charge).
When the potential energy eV.sub.D is greater than a work function of the surface metal layer 4, some electrons of the metal base layer 2 penetrate the potential barrier .phi..sub.A by the tunnel effect. Some of them further pass through the surface metal layer 4 to be emitted into the vacuum space and then flown to the anode. This emission of electrons to the vacuum space takes place only when the potential energy eV.sub.d is greater than the potential barrier .phi..sub.M.
In the conventional electron emitting device described above, however, a diode current I.sub.d which flows from the metal base layer 2 into the surface metal layer 4 is very large. As a result, an emission efficiency .alpha., a ratio of an emission current I.sub.e from the electron emission area 5 to the diode current I.sub.d, is significantly low. FIG. 10 is a graph showing the relationship between the discharge efficiency .alpha. and the thickness D of the surface metal layer 4 in the case of an example of an electron emitting device having an M-I-M structure formed of Al-Al.sub.2 O.sub.3 -Au. As is apparent from this graph, the emission efficiency .alpha. exponentially decreases when the thickness D of the surface metal layer 4 increases. This indicates that energy loss caused by collision of hot electrons with electrons of the surface metal layer 4 is great, and that the possibility of collisions exponentially increases as the layer thickness increases.